The community of Orting, Washington, is built on top of 500-year-old lahar debris from Mount Rainier (rear). Photo: USGS.

May is Volcano Awareness Month in Washington State. On the eve of tomorrow’s anniversary of the May 18, 1980, eruption of Mount St. Helens, DNR Ear to the Ground has featured one of the state’s five active volcanoes each day this week. Today, the spotlight is on Mount Rainier.

Because of its elevation (14,410 feet), massive icecap, glacier-fed valleys, and proximity to Seattle and Tacoma suburbs, Mount Rainier is the most potentially dangerous volcano in the nation — it’s also ranked among the top ten most-most dangerous in the world. According to the US Geological Survey’s Volcano Hazards Program, there hasn’t been a major eruption on Mount Rainier in 1,000 years, but an explosive eruption (a la Mount St. Helens) isn’t the primary concern. Mount Ranier can generate huge lahars — rapidly flowing slurries of mud and boulders — even without an eruption. Avalanches caused by heated rock or volcanic gases can swiftly melt snow and ice and produce torrents of meltwater that pick up loose rock and become a lahar.

In its role as the state’s geological survey, DNR mapped the routes of past Mount Rainier lahars. The most destructive — and most likely — lahar routes are on the mountain’s north and west sides. A lahar here could feed into the Puyallup River valley where cities, towns and housing developments have been built on top of lahar deposits from as recently as 500 years ago.

DNR estimates that a moderately large lahar in the Puyallup River valley would cause $6 billion or more in damages to structures and other property. Large lahars of the past have reached Puget Sound via the Nisqually River Basin, Commencement Bay and Elliott Bay, including the ports of Seattle and Tacoma.

Have you given much thought to the potential of a large lahar from Mount Rainier? Has it influenced your decisions on where to live or work? Join the discussion on DNR’s Facebook page.